Mercury vapor high-pressure discharge lamps usually include an ionizable fill which, besides a noble gas and mercury, also contains metal halides. Lamps with a special composition, namely halides of rare earths, and alkali metals are particularly suitable for general service illumination and especially where the on-off cycling rate is low. Rather low-powered lamps, that is, between 35-400 W and especially 35, 70 or 150 W, are used frequently in double-ended form, in which two pinch seals are provided between which a discharge vessel is located. An external, outer envelope or bulb may also be used. Single-ended lamps, with a single press seal, are also in use, see for example the referenced U.S. Pat. No. 4,658,177.
High-pressure mercury vapor metal halide lamps are available which provide light of various spectral or color characteristics. Basically, three types of lamps are used:
(1) A warm white color, also known as WDL, which corresponds to a color temperature of about 3100 K. Lamps of this type are particularly suitable for interior illumination and used with rather low power ratings, for example about 70 W.
(2) A neutral white color, also known as NDL, corresponding to a color temperature of, typically, about 4300 K. Lamps of this type are also suitable for interior room illumination, and used frequently with intermediate power ratings, that is, for example 150-400 W.
(3) Daylight-simulating color temperature also known as D, which corresponds to a color temperature of at least 5000 K., and used particularly for outdoor illumination, or for accent illumination, and usually provided with higher power ratings, for example 250 W, and above.
The criteria for suitability of such lamps in the general service field are, first of all, a long lifetime, that is, a lifetime in the order of 6000 hours, together with good color rendition, which is expressed in a high color rendering index or, in other words, a high Ra index. The overall color index Ra.sub.8 should, at least, have a value of about 85. Improvement of the individual color rendering indices, for example the color rendering index R9, for rendition of colors in the red spectral region, are of particular importance. It has not been possible heretofore to find a satisfactory compromise between long lifetime and good color rendition of the lamps, especially within the red spectral region. This is particularly true for fills of WDL lamps, that is, which are to emit "warm white" light.
The referenced article "TECHNISCH-WISSENSCHAFTLICHE ABHANDLUNGEN DER OSRAM-GESELLSCHAFT" ("Technical-Scientific Publications of the OSRAM Company"), published by Springer, Heidelberg, 1986, No. 11, pages 11-30, A. DOBRUSSKIN et al: "Halogen-Metalldampfanlagen mit Seltenen Erden" ("Halogen Metal Vapor Lamps with Rare Earths"). describes fills for double-ended metal halide discharge lamps of a power rating between 70 and 250 W for the three above-described light colors. For the D light and NDL, a fill is used which contains mercury and the iodides of dysprosium, holmium, thulium as well as sodium or cesium, respectively, and finally thallium (see also U.S. Pat. No. 3,842,307, Dobrusskin et al, assigned to the assignee of the present application). For WDL, that is "warm white" light, a fill of mercury and iodides or bromides of tin, indium, lithium, sodium and thallium is used, see for example, U.S. Pat. No. 4,171,498, FROMM et al. In a 70 W lamp of a fill suitable for WDL light, no rare earth halides were used, since it has been found that the warm white light (WDL) can be obtained with rare earths, while using aodium and thallium additives, only at wall loadings of the lamp at such a value which would interfere with the lifetime of the lamp by chemical reaction of the fill with the quartz glass, of which the envelope is usually made. Wall loadings which permit the use of rare earth are, for example, over 20 W per cm.sup.2.
On the one hand, it is unsatisfactory to use different fills for different light colors; on the other, the color rendition with respect to the red spectral components of the fills is still unsatisfactory. For WDL light, the color rendering index R9=-90; for NDL, R9=-30. Further disadvantages of these lamps are a relatively low overall color rendering index for WDL lamps, that is, Ra.sub.8 =75, a relatively low light output of about 68 lm/W, and, particularly in case of WDL lamps and NDL lamps, a high degree of spread or scatter of the color temperature of all the three light colors. A sodium-tin fill has an additional disadvantage in that it may lead to increased corrosion of the electrodes, which must be prevented by special and specific construction of the electrodes themselves, see for example "TECHNISCH-WISSENSCHAFTLICHE ABHANDLUNGEN DER OSRAM-GESELLSCHAFT" ("Technical-Scientific Publications of the OSRAM Company"), published by Springer, Heidelberg, 1986, No. 12, pages 65 through 72, D. C. FROMM, "Elektrodenentwicklung fuer kleine halogen-Metalldampflampen" ("Development of Electrodes for Small Halogen Metal Vapor Lamps").
European specification 0 215 524, Meulemans et al, which references the technical-scientific publication of the OSRAM company, a related organization of the assignee of the present application, proposes a solution by using a ceramic discharge vessel with specific geometric relationships between the discharge vessel and the electrodes. This permits the use of indium or rare earth metal halides besides the known suitable components of sodium and thallium. This is an elegant solution--from a theoretical point of view. In actual practice, however, it has been found unsatisfactory since ceramic materials cause problems and are expensive. Problems arise especially in connection with tightness of the seals through which electrodes pass, as well as the development of meltable or flowable glass or glass solder seals which are resistant to halides, and to current supply leads compatible with maintaining tight seals between the ceramic and the specific glass solder or other glass melt substance which is being used.
British Patent 1,370,020, as well as U.S. Pat. No. 3,798,487, Zollweg et al., describe a discharge lamp with a quartz glass bulb which is designed for optimum light output. It retains a fill which, besides mercury, contains a halide of praseodymium (Pr), neodymium (Nd) or cerium (Ce) in a total quantity of 1.4.times.10.sup.-6 to 5.4.times.10.sup.-5 mol/cm electrode spacing, as well as cesium (Cs) halide in a quantity of 3.5.times.10.sup.-7 to 5.4.times.10.sup.-5 mol/cm electrode spacing.
This lamp has an extremely high light output, about 140 lm/W, however has a poor overall color rendering index Ra.sub.8, and particularly a very poor red index R9. The cold spot temperatures of only 600.degree. C. point already to the poor color rendering index data. As far as color rendering is concerned, the lamp has a pronounced green hue, due to the cerium radiation, in a wave length range of between 480-580 nm. A lamp of this type, thus, is not suited for general service illumination, in which optimal color rendition is of primary importance and must be considered ahead of light output.